Where in The Crucible is the commandment "You shall not take the name of the Lord God in vain" broken?
At the end of Act 3, Proctor uses God's name in vain twice as a result of his incredible frustration. Mary Warren has turned on him and he sees no logic or ethical thought process on the parts of Danforth and Parris. He knows all of the accusations of witchcraft are fraud, but the court authorities are too stubborn to entertain this idea. When Danforth prompts Proctor to admit that he is doing the devil's work, a frustrated Proctor responds "God is dead." But Proctor says this as a criticism of the entire court proceedings and of the fraudulent accusations of innocent people. He means that these people are not with God, not doing God's work in this witch hunt.
He says that he and Danforth have the devil's face. He is incriminating everyone in this way, saying they are all guilty of perpetrating the farce that is the witch hunt. Proctor also says that "God damns our kind." That is, God condemns the kind of people who perpetrate fraud and who lack the courage to acknowledge the truth. When he says "you quail now," he means that they have lost the courage (quail) to admit that the witch hunt is based upon lies:
For them that quail to bring men out of ignorance, as I have quailed, and as you quail now when you know in all your black hearts that this be fraud - God damns your kind especially . . .